Nickel-tungsten-aluminum alloy for cathode structure



L. W. KATES NICKEL-TUNGSTEN-ALUMINUM ALLOY FOR CATHODE STRUCTURE FiledApril 29, 1952 \ESRE $5 wqiusq 5 E T NK mm .0 M m F. L

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United States Patent 2,720,458 NICKEL TUNGSTEN ALUMINUM ALLOY FORCATHODESTRUCTURE Leonard WQKa tes, HempsteatL N. Y., assignor toSylvania Electric-Products Inc., a corporation of MassachusettsApplication April- 29, 1952, Serial No."28'4,93 1 6 Claims. or 75-170This inventiotirlafs to'a-hietallic cathode suitable for use inelectrondischarge devices. More particularly, it relates to a high strength'alloy suitable for use under exacting conditions and at hightemperatures and to "a method of making such alloys.

In recentyears it has become desirable to make use of electronic tubesin applications where they are subjected to shock and vibration. Whenused in such applications the tube parts must be especially strong ifthey are not to distort, bend or collapse during tube operation. Themost critical part of such a device is the thermionic emitter or cathodebecause it is subjected to higher temperatures than any other part andtherefore is more easily deformed. Furthermore, it must be capable ofefiiciently emitting electrons at elevated temperatures whencoated withcathode coating materials.

It is therefore an object of this invention to provide a cathode made ofan alloy having especially high hot strength characteristics.

It is a further object of this invention to provide an alloy of theafore-mentioned type which will possess good emission characteristics atoperating temperatures.

It is another object of this invention to provide a method forfabricating alloys of this type.

In the drawings,

Figure 1 is a front elevation partly in section, of a radio tube havinga filamentary cathode.

Figure 2 is a front elevation of an indirectly heated type sleevecathode.

Figure 3 graphically depicts the hot strength characteristics of alloysof this invention.

Figure 4 graphically compares the hot strength characteristics of thealloy of this invention with an alloy of the prior art.

In accordance with this invention it has been found that cathodes havinghigh hot strength and good thermionic emissive qualities can be madefrom ternary alloys containing tungsten, aluminum and nickel.

Figures 1 and 2 of the drawings illustrate the types of cathode of thisinvention.

Figure 1 shows a filamentary type cathode made of a nickel base alloycontaining a small percentage of aluminum and a relatively largepercentage of tungsten. This cathode is shown in a normal tube typehaving a base 12, contact pins 14, and an envelope 16. The elements ofthe tube which can be seen through the cut out portion of the envelope16 include the filament 10, the filament support rod 20, the plate 22,grid support rods 24, grid 26 and the mica spacer 28.

Figure 2 onthe other hand illustrates an indirectly heated type ofcathode consisting of a tubular element 30 made of the nickel base alloycontaining a high percentage of tungsten. The outer surface of thistubular element is coated with an emission material 32 between thebeaded portions 34. The heater 40 which is used with this type ofcathode is shown projecting from the bottom of the tubular element.

2,720,458 Patented Oct. 11, 1955 'ice While it has been recognized forsome time that alloys of nickel and aluminum are extremely well suitedfor the production of cathodes having very good thermionic emissiveproperties and fair hot strength characteristics, no sleeve type orfilamentary type of cathodes have heretofore been made of the ternaryalloy containing nickel, aluminum and tungsten. It has, of course, beenwell recognized that the addition of aluminum to nickel plays animportant role in producing high emission and'long life to the finishedcoated cathode. It has also been suggested that tungsten be used incombination with nickel or in combination with nickel and aluminum inview of its slight reducing action uponbarium oxide which is one of themore common cathode coating materials. For such use amounts up to about4% of tungsten have been recommended.

The alloy of this invention, however, makes use of substantialquantities of tungsten preferably in the neighborhood of 20%. When suchlarge quantitites of tungsten are used in an alloy which may, forexample, contain .75% aluminum and the balance nickel it has been foundthat the alloys thermionic emissive properties are as good as the bestof those presently used in commercial production and considerably betterthan the average. Furthermore, its resistance to deforma tion andrupture at high temperatures during tube processing and operation issuperior to that of the coinmercial nickel base cathode materialspresently being used. This is shown quite clearly in Figure 4 of thedrawings Where the rupture time in minutes is plotted againsttemperature in C. for alloy A as compared to alloy D. The data fromwhich these curves were made was obtained as a result of tests in whichthe filaments were subjected to a constant load of 3800 p. s. i. Thesecurves show, for example, that at 837 C. alloy A has a rupture time 10times larger than alloy D.

Alloy A is one of the alloys of this invention which contains 20% W, 2At% Al and the balance Ni whereas alloy D is one of the betterfilamentary alloys of the prior art containing 1% U, 1.9% A1, 1% W andthe balance Ni.

Although the preferred composition contains 20% of tungsten it has beenfound that alloys having improved hot strength characteristics and highemission can be obtained when the percentage composition of tungstenlies within the range of 5 to 30% tungsten and the percentagecomposition of aluminum lies within the range of .05 to 2% aluminum andthe balance of the alloy being substantially nickel.

The effect on the rupture strength of increasing the amount of tungstenin the alloy can readily be seen in Figure 3 of the drawings in whichthe rupture strength of three different tungsten containing alloys arecompared. Alloy A which exhibits the best properties and the highest hotstrength contains 20% W, 2 At% Al and the balance Ni. Alloy B contains10% W, 2 At% Al, balance Ni, and alloy C contains 5% W, 2 At% Al, andthe balance Ni.

In view of their high content of tungsten these alloys are particularlydifficult to fabricate unless they are prepared under exactingconditions. In accordance with the preferred embodiment of thisinvention the alloys are prepared from high purity nickel, tungsten andaluminum by subjecting these materials to vacuum melting and castingconditions in which the pressure is kept at approximately one micron.When so processed the ingots can be broken down either by hot rolling orforging depending upon whether the finished material is to be made intowire for directly heated filamentary cathodes of the type shown onFigure l or into strip material for fabrication into the indirectlyheated sleeve type cathodes as shown in Figure 2. It has been foundpreferable to keep the temperature of the alloy up to about 1,400 C. forthe first passes in the hot rolling or forging process. The temperaturecan then be subsequently graduated or reduced as the size of thematerial is-reduced. In the preparation of the wire filamentary materialthe alloy is preferably hot worked down to about /s" diameter and thencenterless ground and cold drawn to the proper size. In the preparationof strip material the alloy is hot rolled to a point of .100" and thencold rolled to finished size.

The alloys of this invention when cold drawn have unusually highstrength at room temperature. This makes them adaptable for fabricationinto filaments on automatic machinery without danger of breakage.Furthermore, the annealed material can readily be cold formed intotubular parts.

While the above description and drawings submitted herewith disclose apreferred and practical embodiment of the metallic cathode of thisinvention it will be understood by the specific details of constructionand arrangement of parts as shown and described are by way ofillustration and are not to be construed as limiting the scope of theinvention.

What is claimed is:

1. A cathode structure characterized by high hot strength and composedof a ternary alloy containing 5% to 30% of tungsten, .05% to 2%aluminum, and 68% to 94.5% of nickel.

2. A cathode structure characterized by high hot strength and goodthermionic emission properties comprising a cathode base coated withelectron emissive material, said base being composed in entirety of aternary alloy containing 5%30% tungsten, .O5%2% aluminum, and 68%94.5%nickel.

3. The cathode structure as set forth in claim 2 wherein said structureis a filamentary cathode.

4. The cathode structure as set forth in claim 2 wherein said structureis an indirectly heated cathode.

5. In an electron discharge device, an indirectly heated hollow cathodestructure coated with electron emissive material, said structure beingcomposed of a ternary alloy containing 5%30% tungsten, .O5%2% aluminum,and 68%94.5% nickel.

6. In an electron discharge device, a filamentary cathode structurecharacterized by high hot strength and good thermionic emission, saidstructure including a base coated with electron emissive material, saidbase being formed from a ternary alloy composed of 5%30% tungsten,.O5%2% aluminum and 68%94.5% nickel.

References Cited in the file of this patent UNITED STATES PATENTS859,608 Marsh July 7, 1907 1,634,343 Smith July 5, 1927 2,100,218 KelleyNov. 23, 1937 2,103,267 Mandell Dec. 28, 1937 2,116,788 Haslauer May 10,1938 2,126,746 De Golyer Aug. 16, 1938 2,162,596 Wyman June 13, 19392,194,167 Holladay Mar. 19, 1940 2,410,717 Cox Nov. 5, 1946 2,566,115Bounds Aug. 28, 1951 FOREIGN PATENTS 116,841 Australia Apr. 14, 1943

1. A CATHODE STRUCTURE CHARACTERIZED BY HIGH HOT STRENGTH AND COMPOSEDOF A TERNARY ALLOY CONTAINING 5% TO 30% OF TUNGSTEN, .05% TO 2%ALUMINUM, AND 68% TO 94.5% OF NICKEL.